See FIG. 1. Rigid flanges are generally used in the connection of most steel pipes at present. This rigid flange comprises: Flange Plate 1 welded along the wall of Steel Pipe 3, several Bolt Holes 11 set up on Flange Plate 1, and several Stiffening plates 2 set up on the back of Flange Plate 1. Stiffening plates 2 are welded and fixed to the wall of Steel Pipe 3 and the back of Flange Plate 1 so that Stiffening plates 2 are distributed in the radial direction along Steel Pipe 3. Both pipes are connected by inserting bolts into Bolt Holes 11 on Flange Plate 1. This coupling flange has the following disadvantages when it is used for fatigue dynamic structures: thickness of Flange Plate 1 is limited. The diameter of the bolt is so large that is a little hard to meet the requirement on pre-tightening force of a friction-type high-strength bolt. And its Connecting Welding Seam 7 is subject to the shearing and bending moment effects brought by the pulling force of the bolt. The greater the pulling force of the bolt and the farther away the bolt is from the pipe wall, the greater the bending moment gets. This bending moment will pass through Stiffening plate 2 and impose a radial pressure on the wall of Steel Pipe 3, and thus lead to a large hoop stress of the pipe wall, which has an adverse effect on the stress of Steel Pipe 3. Especially when the bolt is under a great pulling force, the flange will split along the upper and lower flange binding surfaces and the bending rigidity cannot be kept unchanged.
Due to the disadvantages of the rigid flange and because wind power generators are subject to the circulating dynamic effects of the rotation of wind wings, pylons are required have sufficient structural strength and bonding strength to withstand such fatigue dynamic load and to withstand the ultimate load brought by maximum wind pressure. The flange connection parts need to have unchanged bending rigidity to withstand fatigue dynamic load. Therefore said rigid flange cannot be used for the pylon connection of wind power generators.
In order to overcome these disadvantages of the rigid flange described above, a thick flange is usually used to connect the cylindrical pylons of large-scale wind power generators. As shown in FIG. 2, this thick flange comprises Flange Plate 1 and Connecting Pipe 14 sticking out of the back of the vertical Flange Plate 1. Connecting Pipe 14 is welded to End Surface 31 at the coupling end of Steel Pipe 3. Several bolt holes are set up on Flange Plate 1 into which Bolts 16 can be inserted for bolt fixation.
This thick flange provides a thicker Flange Plate 1 and longer bolts. Therefore it is easier to impose and control its pre-tightening force than those of the rigid flange. And it meets the requirement that the bonding strength and rigidity of the cylindrical pylons of large wind power generators should be kept unchanged. But the greatest disadvantages of this thick flange are:
1. Great steel consumption and high material cost. And the flange has to be formed through overall casting. The per-ton cost is 2.5˜3 times of the average cost of ordinary steel structures. And the material used to manufacture this flange is mostly imported. Therefore, this thick flange increases the cost of the structure part of the wind power generators and serious affects and popularization and development progress of wind power generators.
2. Deviation of bolt holes of the thick flange is difficult to deal with and adds to the difficulty of assembling.
To sum up, coupling flanges in the prior art are not very suitable for the connection of cylindrical pylons of large-scale wind power generators and connection of other steel pipe structures that have the same stress features. They cannot provide the reliable bonding strength needed by cylindrical pylons of large-scale wind power generators or ensure that the tensile rigidity remains the same under continuous circulating dynamic loads. Neither can they save steel consumption, reduce the machining difficulty, nor reduce the manufacturing and assembling costs.